1. Field of the Invention
The invention relates to infra-red cameras and, more particularly, to the optical or pre-photocell system and variable apertures for such cameras.
2. Background
Thermal infrared radiation is the emission of photons by any and all objects having a temperature that's above absolute zero. Those emissions may be captured by a thermal infrared camera (alternatively, known simply as an infrared camera), which is well known in the art. An infrared sensitive photocell is central to all infrared cameras. A photocell means any device sensitive to infrared radiation, including a single element detector, a linear array and a two dimensional array of detectors (such as a 2D array of IR sensitive pixels) of various infrared sensitive materials. In infrared cameras the photocell is a two-dimensional array of infrared sensitive pixels. In use, the photocell, highly sensitive to thermal infrared radiation (hereinafter frequently referred to simply as “radiation”) is exposed to radiation emanating from the object or scene being imaged. However, because the camera enclosure is above absolute zero in temperature, the enclosure concurrently emits radiation that is able to reach the photocell. That additional radiation is not desired, since that radiation negatively affects the operation of the camera. To minimize or eliminate that effect, the photocell is enclosed within a cold structure (referred to as a “radiation shield”).
The design of the radiation shield is dictated simply: if an observer were to look out from the photocell, anything the observer could see would emit radiation that would be incident on the camera photocell. In order to block the undesired radiation, the radiation shield must be the only internal camera structure that the photocell is able to “see.” The radiation shield shouldn't emit an excess of radiation. The “cold stop,” which is simply a name for a cooled aperture, provides the only path through the focusing optics for external radiation to reach the photocell. The cold stop size is a compromise between the effectiveness of blocking the unwanted radiation (requiring a small aperture) and excessive vignetting (requiring a large aperture). An ideal cold stop position exists at the exit pupil of the lens. At that location the size of the cold stop is equal to the size of the exit pupil of the front optics, producing 100% effectiveness and no vignetting.
An active cooler, typically, such as a Peltier cooler, is often integrated into the camera to keep the photocell and other components of the infrared camera cool. Typically, in order to control the unwanted radiation seen on the photocell, the cooling system must maintain a low fixed temperature. Ideally, the temperature of the radiation shield is cold enough to produce only a negligible amount of radiation at the photocell. That fixed temperature has a known effect on the photocell and that effect can be removed through image post processing. The photocell is also cooled to improve its radiation sensitivity and reduce the internally generated current, as the higher the temperature of the photocell, the lower its usable dynamic range. However, in more sensitive systems the system must be cooled to as low a temperature as reasonably possible to minimize any unwanted radiation loading. In such systems, several options are available for achieving the necessary cooling, including integrating the cameras into dewars for liquid nitrogen or liquid helium, Stirling cryogenerators, Gifford-McMahon mechanical coolers, and other such devices.
To reduce thermal load on the cooling system, infrared camera designers often place all of the cooled elements inside a vacuum vessel. Within the vacuum vessel, the radiation shield and the photocell are maintained at a low, sometimes cryogenic, temperature, based on the photocell requirements and the desired performance. The vacuum vessel, (if present) often constitutes a camera housing, which also often contains, or provides, a mounting apparatus for the infrared focusing lens. The term “lens” as used herein should be understood to be inclusive of all light collecting devices including refractive or reflective systems.
Thermal infrared cameras must be able to accommodate both hot and cold target objects and scenes, while distinguishing the target from background radiation. Although the thermal control methods described above can allow a camera to be used in a wide variety of thermal scenes, drastic changes in radiation quantities require different camera settings. If the scene is too cool for ideal use with the camera, the camera operator can take a longer exposure of the scene. Doing so may adversely affect the frame rate and may lead to resolution problems if the camera or target is moving. Another solution typically used in the art is to change the electronic gain of the signal from the photocell, even though increasing the gain also increases the noise in the electronic signal. Conversely, in hot scenes, reduced exposure time, reduced signal gain, or a combination of the two can allow an infrared camera to capture the scene. When a very bright event occurs (e.g., an explosion, a launch of a missile) in a scene with a very high dynamic range the photocell could saturate. A method to avoid saturation of the photocell is by reducing the size of the optical aperture. In conventional video camera, the iris mechanism is often coupled to the photocell readout electronics, controlling the iris in response to the radiation intensity.
Apertures and Cold Stops. A cold stop is simply a temperature-controlled aperture. In its most basic form, the cold stop is a fixed aperture, similar to the apertures found in some disposable visible light cameras. Variable diaphragms (hereinafter used interchangeably with the term “iris”) for light cameras, including continuously variable and swappable fixed apertures, have been described in patent art for many years (see e.g., U.S. Pat. No. 24,356 to Miller and Wirsching in 1859, U.S. Pat. No. 582,219 to Mosher in 1897). The variable diaphragm works by allowing more (or less) of the radiation (visible light, in the case of visible light cameras) that reaches the focusing lenses to pass through to the photocell or film. The focusing lens receives radiation and focuses it based on the distance from the radiation source to the lens and the prescription of the lens. The prescription includes the focal length and the f-number. In conventional visible light cameras (and unlike infrared cameras), the aperture is typically built into the compound lens assembly. That aperture then lets pass a certain desired portion of the radiation intercepted by the lens.
With a very large aperture, nearly all of the light arriving at the focusing lens passes through the aperture. By reducing the size of the aperture, the mechanism of the aperture blocks a portion of the light from entering. In typical visible light cameras, the aperture is located at the point where the cone of light from the object is wide, at the pupil or aperture stop; and thus diminishes the light intensity without affecting the image quality. Lenses may have specific aperture requirements, which determine the optimum position and size of the aperture. This is typically a function of the f-number (hereinafter interchangeably also referred to as “f-number”), the focal length of the lens, and the construction.
However, in infrared cameras, the aperture cannot be located in the lens, since the lens is not cooled and the aperture must be cooled so the aperture doesn't radiate onto the infrared photocell. A lens for use with infrared cameras ideally has its exit pupil located far enough behind the lens mount to the camera body, and at the end of the radiation shield, where the cold stop is mounted. The fixed aperture is typically located in the converging path of the light at the end of the radiation shield; that is, between the lens and the focal plane. The aperture thus defines an effective f-number for the system. If the lens f-number matches the fixed aperture f-number the camera is said to be aperture matched. If the lens f-number is smaller, i.e., faster than that of the fixed aperture then some of the incoming radiation is clipped by the aperture, and if the f-number of the lens is larger, i.e., slower, than that of the fixed aperture, the photocell can “see” the mechanical structure of the camera and it receives undesired radiation from the camera structure.
Interchangeable lenses will have different f-number's. Unless the aperture is changed, the f-number of the camera won't match the f-number of most of those lenses. A need thus exists for an adjustable aperture that can match the f-number of interchangeable lenses. That aperture must be placed at the lens' exit pupil location, that is, inside the vacuum enclosure and at the end of the radiation shield, and should be adjustable to match the lens' f-number or the exit pupil size.
As a result, when interchangeable lenses of a different f-number are used with an infrared camera, the system f-number may not match the f-number of the lens. No solution heretofore existed in the prior art to this problem prior to this invention. A variable diaphragm or aperture, however, is able to correct the foregoing situation and match the system f-number to the specific lens in use.
U.S. Pat. No. 6,133,569 (the “'569 patent”) to Shoda and Ishizuya discloses a thermal infrared camera that appears to incorporate the above-mentioned features. The '569 patent further notes the promising idea of using variable diaphragms in thermal feedback infrared cameras, that is, in cameras with thermal sensors controlling cooling elements. Specifically, Shoda and Ishizuya suggest the use of an optically variable diaphragm optionally thermally coupled to the infrared radiation shield, but without providing the reader with any tangible details beyond the basic thought. However, due to the limitations discussed hereafter in regard to cooling the variable diaphragm, the '569 patent does not make possible the use of such a variable diaphragm in an infrared camera.
The use of continuously variable diaphragms or swappable fixed apertures that are used to match interchangeable lenses with different f-number numbers in thermal infrared cameras hasn't been viable because of fundamental packaging and thermal control problems. As earlier described, the aperture must be cooled. While an effectively cooled variable diaphragm is difficult to design, the problem becomes considerably more difficult if the aperture must be kept at cryogenic temperatures and be located inside a vacuum chamber. Within a vacuum chamber, the aperture and the associated drive mechanisms cannot outgas. Depending on the depth of vacuum, this may require a completely dry iris and specially designed lubricants, electrical wiring, motors, and gears. Moreover, the drive mechanism cannot add heat load onto the cooling system, nor allow conductive heat load from the ambient vacuum enclosure to affect the cooling system. Equally important, the aperture must dissipate energy from the radiation that it blocks. These and other considerations for the aperture itself have made implementing a variable diaphragm impossible given the prior art.
Further, with continuously variable diaphragms or interchangeable fixed apertures, there must be some mechanism for changing the size of the aperture. Mechanical, electromagnetic, piezoelectric, or other such control means must be available to change the diaphragm size or interchange the fixed apertures. The control means must be strong enough to operate the variable diaphragm or interchangeable fixed aperture in a timely manner, and either be thermally isolated from the photocell or operate at cryogenic temperatures. If the aperture is in a vacuum, the control means must be small enough to fit within the vacuum chamber or provide some means for transferring mechanical force through the wall of the vacuum chamber. Where such transfer of mechanical force occurs, complex seals must be used to ensure the integrity of the vacuum is un-compromised and that excessive heat is not conducted into the radiation shield.
Aperture control means located in a vacuum chamber require constraints that make their implementation significantly less feasible. First, the materials used in conjunction with the control means cannot outgas, as vaporized materials not only destroy the vacuum that provides the thermal isolation for the cold components, but also condense on the photocell. For that reason, bearings, linings, coatings, winding insulation, and any cements or glues must be eliminated or replaced with a fluorinated polymer or polytetrafluoroethylene based insulation, such as Teflon® brand insulation, or otherwise be coated or manufactured with special non-outgassing materials.
Moreover, the motor control means must also be able to cool itself effectively without the typical convection of heat into air. This means that all heat generated in the motor must be dissipated through conduction to the motor mounting apparatus. The control means must therefore be thermally isolated from the aperture it controls. The motor must incorporate heat-reducing technology, including bipolar drives, low current standby systems, and other such options. Furthermore, the diaphragm control means must not produce electromagnetic interference (EMI) that can distort the electronic signal produced by the photocell. Mechanical or other temperature control means must often also be associated with the motor.
Finally, for control means located in a vacuum, there is an additional potential problem created by high voltage to exposed conductors in the motor apparatus. In extremely low-pressure vacuums, the remaining air molecules subject to high voltage can ionize and current will flow as if the vacuum chamber were an electron tube, creating strong corona effects. These effects are particularly problematic near highly sensitive photocells, so careful insulation is needed on any exposed electric contacts.
An additional packaging problem exists where a variable diaphragm system must fit within the same confines as an existing fixed aperture camera. In these retrofit cases, the entire aperture control means must fit within very small confines that were not designed to accommodate such hardware.
Accordingly, a need exists in the art for a variable diaphragm that overcomes or avoids the above problems and limitations, which constitutes a principal object of the invention.
A further object of the invention is to allow the use of interchangeable optics, including interchangeable compound lenses, in a single infrared camera, by providing a means to match the aperture number (ie. f-stop) of the camera to the aperture number of the lenses.
A still further object of the invention is to retrofit an infrared camera that contains a fixed cold stop aperture for use with a lens that is of a different f-stop number from that of the camera.
Our prior application for U.S. patent Ser. No. 10/250,016, filed May 28, 2003, presently pending, the content of which is incorporated herein, addresses most of the same goals, and describes an invention in a thermal infrared camera that includes a variable aperture. Among other things, the present application describes a variable aperture assembly of improved structure not previously described.